Without limiting the scope of the invention, its background is described in connection with CCD imagers, as an example.
Heretofore, in this field, a CCD imager constructed according to well known techniques comprises a matrix of many cells, each responsive to visible light (or other wavelengths of radiation) for generating a number of electrons based upon the intensity of light impinging on the cells. The electrons collected within each cell of the matrix are then shifted to an output, and serialized, to produce an analog signal which corresponds to a portion of the image. The analog signals can then be amplified and re-transmitted to a CRT or other type of display, or recorded by a video tape recorder, for example.
CCD imagers are typically utilized in video cameras and other types of picture processing equipment to function as a transducer for converting visible images into corresponding electrical signals. CCD imagers adapted for such use are described in detail in U.S. Pat. No. 4,229,752, issued to the applicant and assigned to the same assignee as the present application, and incorporated herein by reference. The light energy reflected from an object is converted into an electrical image in the silicon material of the CCD integrated circuit. A CCD imager typically comprises a matrix of several thousand individual silicon cells which are exposed to the light reflected from the object. The photons which are characteristic of the reflected light enter the silicon cells and generate a number of electron-hole pairs in correspondence to the light intensity. Thus, for light rays having a high intensity, a larger number of electron-hole pairs will be generated than for light rays having a low intensity. Each cell of the CCD array is isolated from the adjacent cells so that the generated electrons within the cells remain associated with each respective cell. In this manner, an electrical image representative of the object image is thus captured. The substrate of the silicon CCD imager is biased to remove the holes of the electron-hole pairs, thereby leaving the captured electrons as the charge which is representative of the object image.
The isolation which typically separates the CCD cells is illustrated in FIG. 1. It comprises a boron diffusion 10 which extends down through the buried channel 12 and contacts the substrate 14 in order to provide lateral isolation between the cells as well as a path for carriers from the p+ virtual gate region 16 to flow to the p-type substrate 14. The requirement in virtual phase CCD imagers that the channel stops 10 reach down to the substrate 14 result in an isolation structure that requires a large amount of the image sensor's surface area, since the channel stop diffusion spreads laterally as well as vertically. Such deep isolation implants are therefore counterproductive with regard to attempts to increase the packing density or increase the number of CCD cells per chip (i.e. higher resolution).
From the foregoing, it can be seen that a need exists for a channel stop isolation scheme for CCD image sensors that will provide adequate lateral isolation between adjacent cells and also provide a path for charge carriers to travel from the virtual gate electrode to the substrate, while occupying a minimum amount of the cell's surface area.